Method for inhibiting growth of bacteria or sterilizing around separating membrane

ABSTRACT

In the invention, the pipe lines around permselective membranes and the surfaces of permselective membranes are intermittently disinfected by adding an inexpensive acid such as sulfuric acid or the like to pre-treated crude water so as to make the water have a pH of 4 or lower. Accordingly, the invention provides a method of surely disinfecting the permselective membranes in membrane separation systems.

TECHNICAL FIELD

The present invention relates to a method for pre-treatment of crudewater in membrane separation, especially for that in reverse osmosis fordesalination or separation, for example, in reverse osmosis fordesalination of seawater, to a method of bacteriostasis or disinfectionfor membranes, and to an apparatus for them.

BACKGROUND OF THE INVENTION

Membrane separation is much used in various fields of desalination ofseawater and saltwater, production of pure water and ultrapure water formedical and industrial use, treatment of industrial drainage, etc. Insuch membrane separation, contamination of the membrane separationapparatus with microorganisms worsens the quality of permeates andlowers the membrane permeability and separability owing to the growth ofmicroorganisms on and around the membranes or to the adhesion ofmicroorganisms and their metabolites onto them. Concretely, theinfluences of microorganisms result in the degradation of the quality ofpermeates, the reduction in the amount of permeates, the increase inrunning pressure or in the increase in pressure loss. In order to evadesuch serious problems, various techniques and methods for bacteriostasisand even disinfecting microbes in membrane separation units haveheretofore been proposed. For example, microbicides are used. Mostgenerally, a chlorine-containing microbicide, of which the effect hasbeen verified and which has the advantages of low cost and easyhandlability, is added to membrane separation units at a concentrationof from 0.1 to 50 ppm or so. One general method of using such amicrobicide comprises adding a microbicide to a pre-treatment zone in amembrane separation apparatus, in which the pre-treated water havingbeen subjected to disinfection with sodium hypochlorite and then toflocculation and filtration is, before being fed into the membraneseparation units, once stored in a tank, and then processed for removingfree chlorine from it through reduction with sodium bisulfite before thesafety filter as disposed in the zone before the membrane treatmentunits.

Chlorine-containing microbicides chemically degrade reverse osmosismembranes. Therefore, when they are used, free chlorine from them mustbe reduced with a reducing agent before they reach reverse osmosismembranes. As the reducing agent, generally used is sodium bisulfite inan amount of from 1 to 10-fold equivalents. The concentration of thereducing agent is determined in consideration of its ability tocompletely remove the remaining microbicide and of the probability ofits reacting with dissolved oxygen in the system being treated. However,even when a membrane separation apparatus is run in a continuous runningmanner according to that method of using such a chlorine-containingmicrobicide, its membrane capabilities are often still worsened, and ithas been found that the method is not always satisfactory fordisinfecting microorganisms in the apparatus. In this connection, it issaid that the chlorine as added in the method oxidizes the organiccarbons existing in the crude water being treated, whereby thethus-oxidized organic carbons are converted into compounds that arereadily decomposed by microorganisms (see A. B. Hamida and I. Moch, Jr.,Desalination & Water Reuse, 6/3, 40-45, 1996), but their theory has notbeen verified as yet. Given that situation, another method for membranedisinfection has been developed, which comprises intermittently addingsodium bisulfite to a membrane separation system generally at aconcentration of 500 ppm. This method has become used in practice, but,in some cases, it is not still effective. Those having tried the methodhave often experienced deposition of microorganisms on permselectivemembranes.

OBJECT OF THE INVENTION

In the conventional pre-treatment method, the pre-treated water havingbeen subjected to disinfection and to flocculation and filtration isstored in a tank for a while, which, therefore, is often contaminatedwith some external contaminants whereby microorganisms much grow in thethus-contaminated, stagnant water to further worsen the quality of thewater. The disinfecting effect of sodium bisulfite to be used in themethod is for removing oxygen from the crude water being processed andto lower the-pH value of the crude water. However, while a permselectivemembrane apparatus is run according to the method, the intermittentaddition of sodium bisulfite to the apparatus is not all the timeeffective for disinfecting the membrane in the apparatus. We, thepresent inventors have studied the reason, and have found that ordinaryaerobic bacteria that grow in a neutral or alkaline condition could beprevented from growing in an anaerobic environment in some degree butcould not be killed in that environment. Having noted it, we havereached the conclusion that the pH depression in the system wherebacteria may live is rather the most effective for disinfecting thebacteria therein. That our conclusion is not contradictory to themicrobiological viewpoint in this respect. On the other hand, we havefurther found that, even when a high concentration of sodium bisulfiteof 500 ppm is added to crude water having a high salt concentration suchas seawater, the pH value of the water system could not be lowered tosuch a degree that ordinary bacteria existing therein could be killed.Therefore, it is understood that sodium bisulfite added to crude waterhaving a lower salt concentration could exhibit its disinfecting effectnot in an anaerobic condition but rather in a lowered pH condition.Accordingly, we have found that adding a high concentration of expensivesodium bisulfite to membrane separation units is not needed fordisinfecting them but merely adding inexpensive sulfuric acid or thelike thereto to lower the pH value in the system around them issatisfactory for disinfecting the units, and that, when the pre-treatedwater is prevented from standing for a while in a tank or the like in awater treatment apparatus, then the growth of microorganisms in theapparatus could be inhibited. On the basis of these findings, we havecompleted the present invention.

DISCLOSURE OF THE INVENTION

The object of the invention can be attained by the constitutionmentioned below. Specifically, the invention provides “a method ofbacteriostasis or disinfection for a permselective membrane in amembrane separation apparatus for water purification, which comprises astep of treating crude water with an acid at a pH of at most 4”, andalso provides a method for purifying water that essentially comprisesthe disinfection method, and an apparatus for the method.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a flow chart showing the constitution of the essential partsof a seawater desalination apparatus.

1: pre-treatment unit

2: reverse osmosis membrane treatment unit

3: post-treatment unit

4: membrane washing unit

6: first duct

7: flocculant feeder

8: sand filter (primary filter)

9: safety filter

10: second duct

11: pH controlling agent feeder

12: third duct

13: microbicide feeder

BEST MODE FOR CARRYING OUT THE INVENTION

The membrane separation unit for the invention is one forwaterproduction, concentration, separation or the like, in crude waterto be treated is fed into a membrane module under pressure and separatedinto a permeate and a concentrate via the membrane. The membrane moduleincludes a reverse osmosis membrane module, an ultrafiltration membranemodule, a precision filtration membrane module, etc. Depending on thetype of the membrane module to be used therein, the membrane separationunit is grouped into a reverse osmosis membrane unit, an ultrafiltrationmembrane unit, and a precision filtration membrane unit. Concretelymentioned herein is a reverse osmosis membrane unit.

The reverse osmosis membrane is a semi-permeable membrane through whicha mixed liquid to be separated partly passes therethrough, for example,a solvent of the liquid could pass through it but the other componentsconstituting the liquid could not. A nanofiltration membrane and a looseRO membrane are also within the scope of a broad meaning of the reverseosmosis membrane. Polymer materials of cellulose acetate polymers,polyamides, polyesters, polyimides, vinyl polymers and the like are wellused for the reverse osmosis membrane. Depending on its structure, themembrane is grouped into an asymmetric membrane having a dense layer onat least one surface, in which the pore size gradually increases fromthe dense layer toward the inside of the membrane or toward the oppositesurface thereof, and a composite membrane having an extremely thinactive layer of a different material formed on the dense layer of theasymmetric membrane. Depending on its shape, the membrane is groupedinto a hollow fiber membrane and a flat sheet membrane. The thickness ofthe hollow fiber membrane and the flat sheet membrane may fall between10 μm and 1 mm; and the outer diameter of the hollow fiber membrane mayfall between 50 μm and 4 mm. The asymmetric or composite, flat sheetmembrane is preferably supported with a substrate of woven fabric,knitted fabric, non-woven fabric or the like. The disinfection method ofthe invention in which is used a mineral acid is effectively applicableto any and every type of reverse osmosis membranes, not depending on thematerial, the structure and the form of the membranes. Typical reverseosmosis membranes to which the invention is applied are, for example,asymmetric membranes of cellulose acetate or polyamide and compositemembranes having an active layer of polyamide or polyurea. Of those, themethod of the invention is especially effective for asymmetric membranesof cellulose acetate and composite membranes of polyamide; and is moreeffective for composite membranes of aromatic polyamide (see JP-A62-121603, 8-138653, U.S. Pat. No. 4,277,344).

The reverse osmosis membrane module is of a practicable form of any ofthe reverse osmosis membranes noted above, for which a flat sheetmembrane is combined with a spiral, tubular or plate-and-frame module,and hollow fiber membranes are bundled up and combined with it. However,the invention does not depend on the form of the reverse osmosismembrane module.

Regarding its capabilities, the reverse osmosis membrane module for usein the invention has a desalination rate of from 98% to 99.9% and awater production rate of from 10 to 25 m³/day in a standardized size of1 m (in length)×20 cm (in diameter), when evaluated for crude seawaterhaving a salt concentration of 3.5% (this is the most general seawaterconcentration) as applied thereto under a pressure of 5.5 MPa and at atemperature of 25° C. for a recovery of 12%; or has a desalination rateof from 98% to 99.9% and a water production rate of from 10 to 25 m³/dayin a standardized size of 1 m (in length)×20 cm (in diameter), whenevaluated for crude seawater having a salt concentration of 5.8% asapplied thereto under a pressure of 8.8 MPa and at a temperature of 25°C. for a recovery of 12%. Preferably, it has a desalination rate of from99% to 99.9% and a water production rate of from 12 to 23 m³/day in astandardized size of 1 m (in length)×20 cm (in diameter), when evaluatedfor crude seawater having a salt concentration of 3.5 as applied theretounder a pressure of 5.5 MPa and at a temperature of 25° C. for arecovery of 12%; or has a desalination rate of from 99% to 99.9% and awater production rate of from 12 to 23 m³/day in a standardized size of1 m (in length)×20 cm (in diameter), when evaluated for crude seawaterhaving a salt concentration of 5.8% as applied thereto under a pressureof 8.8 MPa and at a temperature of 25° C. for a recovery of 12%. Morepreferably, it has a desalination rate of from 99.3% to 99.9% and awater production rate of from 14 to 20 m³/day in a standardized size of1 m (in length)×20 cm (in diameter), when evaluated for crude seawaterhaving a salt concentration of 3.5% as applied thereto under a pressureof 5.5 MPa and at a temperature of 25° C. for a recovery of 12%; or hasa desalination rate of from 99.3% to 99.9% and a water production rateof from 14 to 20 m³/day in a standardized size of 1 m (in length)×20 cm(in diameter), when evaluated for crude seawater having a saltconcentration of 5.8% as applied thereto under a pressure of 8.8 MPa andat a temperature of 25° C. for a recovery of 12%. The reverse osmosismembrane module having a spiral form comprises other members of awater-feeding duct, a permeate-taking out duct and others, in which theother members may be made of any materials. Especially preferably, themodule is at least partly so designed that it is applicable tohigh-concentration crude water having a salt concentration of at least3.5% and is applicable to high-pressure operation at a pressure of atleast 7.0 MPa (see JP-A 9-141060, 9-141067).

The running pressure to be applied to the reverse osmosis membrane unitfor use in the invention may fall between 0.1 MPa. and 15 MPa, and mayvary depending on the type of the crude water to be treated in the unitand on the unit running mode. For example, crude water having a lowosmotic pressure, such as saltwater, ultrapure water or the like may beapplied to the unit under a relatively low pressure. However, fordesalination of seawater, for treatment of drainage and for recovery ofuseful substances, the crude water to be treated is applied to the unitunder a relatively high pressure.

The temperature at which the reverse osmosis membrane unit is run mayfall between 0° C. and 100° C. If it is lower than 0° C., the crudewater being treated will be frozen so that the unit could not be run;but if higher than 100° C., the crude water applied to the unit willvaporize and could not be well treated.

The recovery in the separation unit may be suitably determined within arange of from 5 to 100%, depending on the mode of running the unit forseparation and on the type of the unit. The recovery in the reverseosmosis membrane unit may be suitably determined within a range of from5 to 98%. For this, however, the pre-treatment condition and the unitrunning pressure must be taken into consideration, depending on theproperties of the crude water to be treated and the concentrate from it,on their concentrations, and even on the osmotic pressure in the unit(see JP-A 8-108048. For example, for seawater desalination, the recoveryin the unit having an ordinary efficiency may fall between 10 and 40%,but that in the unit having a high efficiency may fall between 40 and70%. For saltwater desalination or for ultrapure water production, theunit may be driven to attain a recovery of at least 70%, for example,from 90 to 95%.

Regarding its configuration, the reverse osmosis membrane module may bedisposed in a single stage, but if desired, plural reverse osmosismembrane modules may be disposed in series or in parallel relative tothe running direction of the crude water to be treated therewith. It isdesirable to dispose plural reverse osmosis membrane modules in seriesrelative to the running direction of the crude water to be treatedtherewith, as the crude water could be contacted with the membranemodules for a long period of time. In that condition, the method of theinvention produces better results. Where plural membrane modules aredisposed in series relative to the crude water running therethrough, thepressure to the crude water may be suitably increased between theadjacent stages of the modules. The pressure increase may be effectedwithin a range of from 0.1 to 10 MPa, for which a pressure pump or abooster pump may be used. In addition, plural reverse osmosis membranemodules may also be disposed in series relative to the running directionof the permeate passing through them. This method is favorable when thequality of the permeate is desired to be improved further or when thesolute in the permeate is desired to be recovered. Where plural reverseosmosis membrane modules are connected in series relative to thepermeate passing through them, a pump may be disposed between theadjacent membrane modules via which the pressure to the permeate may beincreased, or the permeate having been excessively pressurized in theprevious stage may be subjected to the next membrane separation underback pressure thereto. In that condition where plural reverse osmosismembrane modules are connected in series relative to the permeatepassing through them, an acid feeder is disposed between the adjacentmembrane modules so that the membrane module in the latter stage couldbe disinfected with an acid from it.

The fraction of the crude water not having passed through the reverseosmosis membrane unit is taken out of the membrane module, and this is aconcentrate from the crude water. Depending on its use, the concentrateis further treated and its waste is discarded, or may be againconcentrated in any desired method. A part or all of the concentrate maybe circulated to and combined with the crude water being treated in theunit. Also depending on its use, the fraction of the crude water havingpassed through the membrane may be discarded as it is, or may bedirectly used as it is, or a part or all of the fraction may becirculated to and combined with the crude water being treated in theunit.

In general, the concentrate formed in the reverse osmosis membrane unithas pressure energy, and it is desirable to recover the energy forreducing the unit running cost. For this, an energy recovery unit may befitted to a high-pressure pump as disposed in any stage, via which thepressure energy of the concentrate could be recovered. Preferably, aspecific, turbine-type energy recovery unit is disposed before or afterthe high-pressure pump or between the adjacent modules, via which thepressure energy of the concentrate could be recovered. Regarding itscapabilities, the membrane separation unit could treat water at a rateof from 0.5 m³/day to 1,000,000 m³/day.

In the invention, the crude water to be treated shall have a pH value ofat most 4, and the pH control is extremely important for surelydisinfecting the permselective membranes used. In particular, when crudeseawater is treated through membrane filtration, the effect of theinvention is remarkable. The pH value at which microorganisms shall dieis specific to the type of microorganisms. For example, the lowermostlimit of the pH value at which Escherichia coli could grow is 4.6, butEscherichia coli shall die at a pH of 3.4 or lower. On the other hand,many types and varieties of microorganisms exist in seawater, and theyshall die at different pH values. However, in the invention, whenseawater containing such many types and varieties of livingmicroorganisms is kept at a pH of at most 4 for a predetermined periodof time, from 50 to 100% of those microorganisms could be killed. Forthis, preferred is an acidity of pH of at most 3.9, and more preferredis an acidity of pH of at most 3.7. In seawater containing many typesand varieties of living microorganisms, some of those microorganismswill be resistant to acids. Even in that case, at least 98% ofmicroorganisms therein could be killed when seawater is kept at a pH of2.6 or lower for a predetermined period of time. Therefore, the methodof the invention could generally produce better results when the crudewater to be treated therein is kept at a pH of at most 4 for apredetermined period of time and is occasionally kept at a pH of 2.6 orlower. For the desired pH control in the method, generally employed isan acid. The acid may be any of organic acids and inorganic acids. Fromthe economical aspect, however, sulfuric acid is preferred. The amountof sulfuric acid to be added is proportional to the salt concentrationin the crude water to be treated. For example, adding 50 ppm of sulfuricacid to a physiological saline solution (having a salt concentration of0.9%), which was subjected to pressure disinfection (at 120° C. for 15minutes), could lower the pH of the solution to 3.2. However, addingeven 100 ppm of sulfuric acid to each of three samples of seawatercollected in different places and one sample of commercially-availableartificial seawater (having a salt concentration of 3.5%), which wereall subjected to pressure disinfection (at 120° C. for 15 minutes),lowered the pH of those seawater samples only to the range between 5.0and 5.8. This will be probably because the pH of those seawater sampleswould greatly vary essentially depending on the M alkalinity of theseawater. To further lower the pH of those seawater samples, adding atleast 120 ppm of sulfuric acid thereto is needed for attaining pH of 4or lower, or adding at least 250 ppm of sulfuric acid thereto is neededfor attaining pH of 2.6 or lower. In consideration of the economicalaspect and of the influence on the equipment including pipe lines, theamount of the acid to be added will be preferably 400 ppm, morepreferably 300 ppm. Further increasing the concentration of sulfuricacid added to the samples of seawater and artificial seawater notedabove to 150 ppm, 200 ppm, 250 ppm and 300 ppm resulted in the reductionin the pH change in the samples of from pH 3.2 to 3.6, from pH 2.8 to2.9, pH 2.6, and pH 2.4, respectively, in accordance with the increasein the concentration of the acid added. If the pH of seawater to betreated is all the time kept at 2.6, all bacteria includingacid-resistant bacteria in seawater will be almost completely killed.However, the proportion of acid-resistant bacteria in seawater is small.Therefore, in the method of the invention, it is desirable that seawateris disinfected generally at a pH of from 2.7 to 4, but occasionally at apH of 2.6 or lower for disinfecting acid-resistant bacteria therein, forsaving the costs of the chemicals to be used and for reducing theinfluences of the chemicals used on the pipe lines.

For disinfecting the membranes in the method of the invention, an acidmay be intermittently added to the crude water after the crude water hasbeen pre-treated and before it is applied to the membrane module. Whereplural membrane modules are disposed in series relative to the runningdirection of the permeate passing therethrough, an acid for membranedisinfection may be intermittently added to the site between theadjacent membrane modules so as to disinfect the latter membrane module.The time for the acid addition and the frequency of the acid additionshall greatly vary, depending on the site to which the acid is added andthe condition for the acid addition. For example, the acid addition maybe effected over a period of from 0.5 to 2.5 hours, once a day, a weekor a month. The same shall apply also to the case of disinfectingacid-resistant bacteria. However, when the acid addition is directed toattaining two different pH conditions in two steps, it is desirable thatthe step of acid treatment for a pH range of from 2.7 to 4 (step A) iseffected at a frequency of once in a period of from one day to 30 daysand the step of acid treatment for a pH range of at most 2.6 (step B) iseffected at a frequency of once in a period of from 2 days to 180 days.When the step A and the step B are effected plural times within apredetermined period of time, it is desirable that the ratio of thetotal time for the step A (TA) to that for the step B (TB), TA/TB, fallsbetween 1/100 and 100/1. In consideration of the process cost and theapparatus durability, it is more desirable that the ratio TA/TB fallsbetween 1 and 100. The operation for the step A may be directly switchedto that for the step B, and vice versa. However, it is desirable thatcrude water not subjected to pH control or crude water having a pH offrom 6.5 to 7.5 is fed to the system between the step A and the step B.The crude water not subjected to pH control or that having a pH of from6.5 to 7.5 in this case may be treated in ordinary membrane separationand the resulting permeate or concentrate may be used for its intrinsicpurposes. The amount of the additional crude water may vary, dependingon the decrease in the amount of the permeate, on the increase in thenumber of living bacteria in the concentrate and in the organic carboncontent of the concentrate, and on the increase in the membranepressure. Where the membrane separation method of the invention iscarried out in a discontinuous manner, the membranes may be dipped in anacid for disinfecting them while the apparatus is stopped.

The disinfection method of the invention may be combined with any otherdisinfection with chlorine or the like.

The membrane disinfection method of the invention is applicable not onlyto membrane separation units but also to water separation systems partlycomprising membrane separation units.

For example, the invention is applicable to the constitution of thefollowing systems.

A. Water-intake Apparatus

This is an apparatus for taking crude water, and generally compriseswater-intake pumps, chemical feeders, etc.

B. Pre-treatment Apparatus Connected with Water-intake Apparatus

This is an apparatus for pre-treating crude water to be fed to apermselective membrane apparatus, in which the crude water is purifiedto a predetermined degree and which comprises, for example, thefollowing units as connected in that order.

B-1: Flocculation and filtration unit.

B-2: Polishing filtration unit.

In place of B-1 and B-2, an ultrafiltration unit or a precisionfiltration unit may be used.

B-3: Chemical feeders for feeding flocculants, microbicides, pHcontrolling agents, etc.

C: Optional Intermediate Tank Connected with Pre-treatment Apparatus

This is for controlling the water level and for buffering the quality ofwater.

D: Filter Connected with the Intermediate Tank C, or Directly withPre-treatment Apparatus in the Absence of the Intermediate Tank C

This is to remove solid impurities from water to be fed into themembrane separation apparatus.

E. Membrane Separation Apparatus

This comprises high-pressure pumps and permselective membrane modules.In this, plural membrane separation units may be disposed in series orin parallel. Where they are connected in series, a pump may be disposedbetween the adjacent membrane separation units, via which the waterpressure to be applied to the latter unit may be increased.

F. Post-treatment apparatus connected with membrane separation apparatusat the outlet through which the permeate runs out. For example, thiscomprises any of the following units.

F-1: Degassing unit, which is for decarbonation.

F-2: Calcium tower.

F-3: Chlorine feeder.

G. Post-treatment apparatus connected with membrane separation apparatusat the outlet through which crude water runs out. For example, thiscomprises any of the following units.

G-1: Unit for treating crude water having pH of 4, for example,neutralization unit.

G-2: Drainage.

H. Any other optional apparatus for treating waste water.

In these systems, pumps may be disposed in any desired zone.

It is desirable that chemicals or chemical solutions for making crudewater have a pH of at most 4 are added to the systems in the waterintake apparatus A or in the pre-treatment apparatus B, or before thepre-treatment apparatus B, or before the filter D, or after the filterD.

For further enhancing the effect of the invention, it is desirable thatthe acid feeder is automatically controllable. For example, the acidfeeder is preferably equipped with a pump capable of controlling theamount of the acid to be fed. For controlling the acid amount, it isalso desirable to dispose a pH meter for measuring the pH of the crudewater and the concentrate in any desired site of the system. Forcontrolling the intermittent acid addition, it is still desirable todispose a timer in the system. Further preferably, the system isequipped with an automatic controller for ensuring automatic running ofthe system.

The members constituting the apparatus of the invention, such as pipes,valves and others are preferably made of materials that will be notdegraded at a pH of 4 or lower. For example, usable are stainless steelmembers, inner surface-coated members, resin members, etc.

Controlling the pH of crude water to be at most 4 ensures gooddisinfection of permselective membranes, and, in addition, thethus-controlled crude water is further effective for removing scale inpipe lines. Sodium bisulfite will have to be added for preventingpermselective membranes from being degraded by chlorine oxides and thelike, and its amount will have to be increased when the microorganisms(including sulfur bacteria, etc.) having adhered onto the membranesincrease or when metal salts also having adhered thereonto increase.However, in the invention where the crude water to be treated isacidified, the amount of sodium bisulfite to be added in that conditioncould be significantly reduced.

The method of the invention is favorable to membrane separation,especially to that of aqueous solutions. In particular, it is especiallyeffective for liquid-solid separation and liquid concentration withprecision filtration membranes, for impurity separation and permeateconcentration with ultrafiltration membranes, and for solute separationand permeate concentration with reverse osmosis membranes. Morespecifically, the invention is effective for desalination of seawater,desalination of saltwater, production of industrial water, production ofultrapure water and pure water, production of medical water,concentration of food and drink, purification of city water, qualityimprovement in city water. In addition, for separating and concentratingorganic substances that are readily degraded with conventional oxidizingmicrobicides, the method of the invention is effective. According to themethod of the invention, such organic substances are not degradedthrough oxidation, and could be surely concentrated and recovered. Inproducing water to drink in the invention, trihalomethanes that may beformed in disinfection with chlorine are not formed.

For disinfection of crude water in pre-treatment, in general, achlorine-containing microbicide is continuously added to it, as somentioned hereinabove. According to the method of the invention, crudewater is almost completely disinfected so far as acid-resistant bacteriado not grow in it. Since the microbicide chemically degrades reverseosmosis membranes, a reducing agent such as typically sodium bisulfiteis added to crude water before the membrane separation unit. However, inthe crude water from which the remaining microbicide is removed in thepre-treatment step, microorganisms could easily grow. In addition, ithas been, clarified that crude seawater to which no microbicide is addedcontain specific groups of microorganisms from among many types andvarieties of microorganisms, and some of those microorganisms existingin the non-disinfected, crude seawater are acid-resistant ones. Further,it has also been clarified that, when a satisfactory amount of areducing agent such as typically sodium bisulfite is not added to crudewater to which a chlorine-containing microbicide has been added, themicrobicide remaining in the crude water could not be completely removedfrom it, but if the reducing agent is added too much, some types ofmicroorganisms will rather grow in the crude water. For these reasons,it is desirable not to add a chlorine-containing microbicide to thecrude water to be treated according to the method of the invention. Ifso, however, microorganisms will grow in the crude water in thepre-treatment step. The problem could be solved by intermittently addinga microbicide and a reducing agent to crude water. In that condition,the microorganisms having adhered to and deposited on the inner walls ofpipes and filtration tanks in the pre-treatment step in the absence of amicrobicide could be killed by the microbicide intermittently addedthereto. The intermittent addition mode is preferable, as not degradingthe membranes. The interval for the intermittent microbicide additionmay be determined depending on the quality of the crude seawater to betreated and on the condition of the microorganisms growing therein. Forexample, a microbicide may be added once at intervals of from 1 day to 6months, and the time for the addition may be from 30 minutes to 2 hoursor so. Depending on the interval, the membranes may be disinfectedaccording to the method of the invention. The intermittent,chlorine-containing microbicide addition significantly reduces thetreatment cost, which is ensured for the first time only by the membranedisinfection method of the invention but not at all by the conventionalmembrane disinfection method of using a high concentration of sodiumbisulfite. This is because the conventional membrane disinfection methodis not satisfactory for completely disinfecting microorganisms. Forpreventing the adhesion and deposition of microorganisms in the absenceof a microbicide, and for enhancing the disinfection effect of theinvention, one favorable system is mentioned below, as illustrated inFIG. 1.

The system of FIG. 1 is for desalination of seawater, which comprises apre-treatment unit 1, a reverse osmosis unit 2, a post-treatment unit 3,and a membrane washer unit 4. The pre-treatment unit 1 comprises aflocculant feeder 7 through which a flocculant solution is added to theseawater (crude water) running through the first duct 6; a sand filter 8which is a primary filter means; a safety filter 9 which is a secondaryfilter means; a pH-controlling agent feeder 7 through which apH-controlling mineral acid solution is added to the primary filtraterunning through the second duct 10; and a microbicide feeder 13 throughwhich a microbicide solution is added to the secondary filtrate runningthrough the third duct 12.

The first duct 6 is connected with the water intake pump 14 and with thesand filter 8; the second duct 10 is connected with the sand filter 8and With the safety filter 9; the third duct 12 is connected with thehigh-pressure pump 15 and with the first-stage membrane module 17 in thereverse osmosis unit 2.

Accordingly, seawater can be fed to the sand filter 8 by driving thewater intake pump 14, and the secondary filtrate can be pressurized to ahigh degree and fed to the reverse osmosis membrane unit 2 by drivingthe high-pressure pump 15. In this step, ferric chloride having apredetermined concentration is added to the seawater through theflocculant feeder 7 via the duct 18, while sulfuric acid is addedthereto through the pH-controlling agent feeder 11 via the duct 19; anda sulfuric acid solution is intermittently added thereto through themicrobicide feeder 13 via the duct 20. The duct 20 may be connected withthe duct 12, or, as the case may be, the pH-controlling agent feeder 11and the microbicide feeder 13 can be integrated into one unit.

From the tank 22 of the flocculant feeder 7, a ferric chloride solutionis fed into the crude seawater being-treated, by driving the pump 21;and from the tank 24 of the pH-controlling agent feeder 11, sulfuricacid is fed into the crude, seawater by driving the pump 23.

In the system of FIG. 1, the pipe line from the water intake pump 14 tothe first-stage membrane module 17 in the reverse osmosis membrane unit2 is a closed pipe line. In other words, this is not a so-called openpipe line where the crude water being treated is temporarily stored in atank, as in a conventional system, but is a non-open, closed pipe line.The system of the invention may comprise a crude water tank, a sandfiltration tank and a feeding pump, in which, however, the pipe linefrom the water intake unit to the reverse osmosis membrane module ispreferably a non-open, closed pipe line.

In the non-open, closed pipe line, the crude water being treated isprotected from being contaminated with any external contaminants, andcan be treated continuously. The flow rate change after thehigh-pressure pump 15 may be prevented by controlling the flow rate inthe units constituting the pre-treatment unit 1. In that condition, thecrude water can be kept all the time flowing through the pipe line, notstanding anywhere in the pipe line, and can be treated continuously inthe line. The sand filter 8 can be driven always stably.

In the pre-treatment unit, a polishing filter may be disposed after thesand filter. If desired, a UF or MF membrane having a pore size of from0.01 to 1.0 μm may be used in place of the sand filter or the polishingfilter or in place of the two.

In the system illustrated, the crude water being treated does not stayin a tank or the like, and therefore adhesion and deposition ofmicroorganisms in the pipe line even in the absence of a microbicidetherein can be prevented. Therefore, the disinfecting effect of theinvention can be enhanced in the system of that type.

EXAMPLES

The invention is described concretely with reference to the followingExamples, which, however, are not intended to restrict the scope of theinvention. In these Examples, the disinfecting effect is represented bythe number of living microorganisms, the pressure loss in membranemodules and the consumption of sodium bisulfite (SBS).

Reference Example 1

A predetermined amount of a suspension of living cells of Escherichiacoli K12 IFO 3301 was added to a physiological saline solution (having asalt concentration of 0.9%) that had been subjected to pressuredisinfection (at 120° C. for 15 minutes) and then to pH control withsulfuric acid added thereto, and kept at 20° C. for a predeterminedperiod of time, and the survival rate of the cells was obtained bydividing the number of the living cells still remaining in the solutionby the number of the cells added to the solution. As a result, thesurvival rate of the cells was not lower than 90% when the solution towhich had been added 10 ppm of sulfuric acid and which had a pH of 4.7was kept under the condition for 2.5 hours. However, the survival rateof the cells in the solution having a pH of 3.2, to which had been added50 ppm of sulfuric acid, was 90% after kept for 0.5 hours, 20% afterkept for 1 hour, and 1% or lower after kept for 2.5 hours. When 100 ppmof sulfuric acid was added to the solution, the survival rate of thecells in the solution was 1% or lower after 0.5 hours.

Reference Example 2

To a commercially-available, 3.5% artificial seawater that had beensubjected to pressure disinfection (at 120° C. for 15 minutes) and thento pH control with sulfuric acid added thereto, added was apredetermined amount of the same suspension of Escherichia coli cells asin Example 1, or a predetermined amount of a suspension of a soliddeposit on a reverse osmosis membrane having been used in seawaterdesalination, or a predetermined amount of un-identified bacteria asseparated from the solid deposit suspension, of which the number was thelargest among all bacteria separated from the suspension. Then, eachseawater was kept as such at 20° C. for a predetermined period of time,and the survival rate of the cells therein was measured. The data areshown in Table 1. For comparison, 500 ppm of sodium bisulfite was addedin place of sulfuric acid, and the data obtained are also shown inTable 1. From the data in Table 1, it is understood that the cells inthe seawater were killed to an extremely high degree when the seawaterwas kept at a pH of 4.0 or lower for a period of 0.5 hours or longer.

TABLE 1 Survival Rate (%) Concen- Suspension of Cells from Add- trationDeposit on Deposit on itive (ppm) Time (hr) pH E. coli Membrane MembraneNone — 2.5 8.5 100 100 100 Sodium 500 2.5 5.9 98 90 86 Bisul- fiteSulfuric 100 2.5 5.1 107 60 81 Acid Sulfuric 120 0.5 4.0 105 37 AcidSulfuric 120 2.5 4.0 93 15 Acid Sulfuric 150 0.5 3.3 <1 <1 <1 AcidSulfuric 200 0.5 2.9 <1 <1 <1 Acid Sulfuric 300 0.5 2.5 <1 <1 <1 Acid

Example 1

Two membrane separation units each having a reverse osmosis membrane ofpolyamide were driven for seawater desalination through reverse osmosisfiltration to produce fresh water. To one of the two units, crudeseawater having been pre-treated and subjected to pH control to have apH of from 3.5 to 4.0 with sulfuric acid added thereto was appliedeveryday for a period of 30 minutes, a day. In that condition, the twounits were continuously driven for 1 month. As a result, the pressureloss in the unit to which no sulfuric acid had been added increased, butthe pressure loss in the other unit to which sulfuric acid had beenadded did not change. While the units were driven under the condition,the number of the living cells in the concentrate having passed througheach unit was counted. As a result, the number of the living cells inthe concentrate in the unit that had been subjected to the sulfuric acidtreatment was lowered to {fraction (1/100)} or less, as compared withthat of the living cells in the concentrate in the other unit notsubjected to the sulfuric acid treatment.

Example 2

Crude seawater, in which the number of the living cells was 200 cells/mlas counted with an agar plate counter, was applied to a membraneseparation unit having a reverse osmosis membrane of polyamide, in whichthe crude seawater was subjected to reverse osmosis separation. In thepre-treatment unit before the membrane separation unit, achlorine-containing microbicide was continuously added to the crudeseawater so that the remaining chlorine concentration therein could be 1ppm. Just before the reverse osmosis membrane module in the separationunit, sodium bisulfite was added to the crude seawater being treated.The amount of sodium bisulfite added was so controlled that theremaining sodium bisulfite concentration in the brine to be taken awaythrough the module could be at least 1 ppm. The consumption of sodiumbisulfite was 5 ppm in the initial stage. However, after the system wascontinuously driven for 10 days, the consumption of sodium bisulfiteincreased up to 35 ppm. Within those 10 days, the pressure loss in themembrane module increased by about 0.01 MPa.

Next, crude seawater having been subjected to pH control to have a pH offrom 3 to 4 with sulfuric acid added thereto was passed through themembrane separation unit for a period of 30 minutes a day. As a result,the consumption of sodium bisulfite decreased to 8 ppm. In this case,the pressure loss increased by 0.01 MPa, as compared with the originalvalue, and was kept later as such.

Example 3

Crude seawater, in which the number of the living cells was 200,000cells/ml as counted with an agar plate counter, was applied to amembrane separation unit having a reverse osmosis membrane of polyamide,in which the crude seawater was subjected to reverse osmosis separation.In the pre-treatment unit before the membrane separation unit, achlorine-containing microbicide was continuously added to the crudeseawater so that the remaining chlorine concentration therein could beat least 1 ppm, and 6 ppm of a de-chlorinating agent of sodium bisulfitewas also continuously added thereto. In the membrane separation unit,500 ppm of sodium bisulfite was added to the crude seawater over aperiod of 1 hour a week. After the system was driven for about 1 month,the, pressure loss in the membrane separation unit increased by about0.02 MPa, as compared with the initial value.

The same crude seawater was treated in the same system as above. In thiscase, however, 1 ppm of the chlorine-containing microbicide was,intermittently added to the crude seawater in the pre-treatment unitover a period of 1 hour a day, and 6 ppm of sodium bisulfite was theretoover a period of 3 hours a day; and crude seawater having been subjectedto pH control to have a pH of 4 with sulfuric acid added thereto wasapplied to the membrane separation unit over a period of 1 hour a day.After about 1 month, the pressure loss in the membrane separation unitchanged little.

Example 4

The same crude seawater was pre-treated in the same manner as in thelatter process of Example 3. Then, the crude seawater was treated in thesame membrane separation unit as in Example 3. In this, however, themembrane in the unit was not disinfected, and the system was driven for50 days. As a result, the pressure loss in the membrane separation unitincreased by 0.03 MPa. After this stage, crude seawater having beensubjected to pH control to have a pH of 3 with sulfuric acid addedthereto was applied to the membrane separation unit, over a period of 1hour a day. After 8 days, the pressure loss decreased by 0.015 MPa.Next, the system was driven for further 20 days without disinfecting themembrane separation unit. As a result, the pressure loss increased by0.02 MPa. After this stage, crude seawater having been subjected to pHcontrol to have a pH value of 4 with sulfuric acid added thereto wasapplied to the membrane separation unit, over a period of 1 hour a day.After 12 days, the pressure loss again decreased by 0.012 MPa.

Example 5

In a system comprising a pre-treatment unit and a membrane separationunit having a module of reverse osmosis membrane of polyamide, crudeseawater was desalinated through reverse osmosis filtration into freshwater. In the pre-treatment unit, continuously added was chlorine to thecrude seawater so that the remaining chlorine concentration thereincould be 1 ppm, and sodium bisulfite was added to the crude seawaterbefore the reverse osmosis membrane module. The amount of sodiumbisulfite added was so controlled that the remaining sodium bisulfiteconcentration in the brine to be taken away from the reverse osmosismembrane module could be at least 1 ppm. After the system was driven,the consumption of sodium bisulfite increased. After 10 days, theconsumption of sodium bisulfite (this is obtained by subtracting theamount of sodium bisulfite remaining in the brine from that added to thecrude seawater) reached 21 ppm. After this, crude seawater having beensubjected to pH control to have a pH of 2.5 with sulfuric acid addedthereto was passed through the membrane unit over a period of 30 days onday 1, day 2 and day 10, and crude seawater also having been subjectedto pH control to have a pH of 3 with sulfuric acid added thereto waspassed therethrough over a period of 30 minutes on day 14 and day 27. Inthis stage, the consumption of sodium bisulfite decreased to 10 ppm.

TABLE 2 Pre-treatment Disinfection RO Disinfection Number ofMicrocide*/−Reducing Time Time Living Cells Agent (min/day) Microbicide(min/day) pH Example 1 None — Sulfuric Acid 30 3.5 to 4   None — 6.5Example 2 200 Cl-containing micro- continuous None — bicide/−NaHSO₃addition Sulfuric Acid 30 3 to 4 (1 ppm excessive each) Example 3200,000 Cl-containing micro- continuous NaHSO₃ 60/7 bicide/−NaHSO₃addition 500 ppm (60 minutes a week) (1 ppm excessive/6 ppm) 60/180Sulfuric Acid 60 4.0 Example 4 200,000 Cl-containing micro- 60/180 None— bicide/−NaHSO₃ Sulfuric Acid 60 3.0 (1 ppm excessive/6 ppm) None —Sulfuric Acid 60 4.0 Example 5 Cl-containing micro- continuous None —bicide/−NaHSO₃ addition Sulfuric Acid 30 2.5 (1 ppm excessive each) 3  Results Running Period Increase in Number of NaHSO₃ (days) Pressure LossLiving Cells Consumption Example 1  1 to 30 No Decreased to Yes 1/100 orless Example 2 0  5 ppm  1 to 10  0.01 MPa 35 ppm 11 to 11  0.01 MPa  8ppm Example 3  1 to 30  0.02 MPa  1 to 30 No Example 4  1 to 50  0.03MPa 51 to 58 −0.015 MPa 59 to 78  0.020 MPa 79 to 90 −0.012 MPa Example5  1 to 10 21 ppm 11, 12 & 20th 10 ppm 14, 27th *: *1 ppm excessive*means that the remaining chlorine concentration in the crude seawater astreated in the pre-treatment unit was 1 ppm, and that the reducing agenthaving remained in the brine as taken away from the reverse osmosismembrane module was 1 ppm.

Comparative Example 1

1% of seawater was added to commercially-available, 3.5% artificialseawater having been subjected to pressure disinfection (at 120° C. for15 minutes), and the pH of the resulting seawater mixture was measuredto be 8.5. After having been kept at 20° C. for 2 hours, 0.1 ml of theseawater mixture was applied onto an agar medium for marine bacteria, ofwhich the pH value had been controlled to be 7, and then kept warmed at20° C. After incubated for a few days, the medium had 200 coloniesformed thereon.

Reference Example 3

1% of seawater was added to commercially-available, 3.5% artificialseawater having been subjected to pressure disinfection (at 120° C. for15 minutes) and then to pH control with 200 ppm of sulfuric acid addedthereto. The pH of the resulting seawater mixture was 2.8. After havingbeen kept at 20° C. for 2 hours, 0.1 ml of the seawater mixture wasapplied onto an agar medium for marine bacteria, of which the pH valuehad been controlled to be 7. After incubated for a few days, the mediumhad 3 colonies formed thereon. The data in this Reference Example 3 areshown in Table 3, along with those in Comparative Example 1. Themicrobes having formed the colonies on the agar media are acid-resistantmicrobes that could not be killed at a pH of 2.8, and it is believedthat 1.5% of such acid-resistant microbes existed in the seawater testedherein.

TABLE 3 Condition Number of for Treatment Colonies Formed ComparativeExample 1 pH 8.5, for 2 hours 200 Reference Example 3 pH 2.8, for 2hours  3

Reference Example 4

To commercially-available 3.5% artificial seawater having been subjectedto pressure disinfection (at 120° C. for 15 minutes) and then to pHcontrol with sulfuric acid added thereto, added, were a predeterminedamount of un-identified acid-resistant bacteria (3 strains in all) thathad been obtained in Example 7, and kept at 20° C. for a predeterminedperiod of time. Then, the survival rate of the bacteria in thepH-controlled artificial seawater was obtained, and the data are shownin Table 4. From Table 4, it is understood that the seawater is welldisinfected when kept at a pH of 2.6 or lower for a period of 0.5 hoursor longer.

TABLE 4 Survival Rate (%) Concen- Acid- Acid- Acid- Add- tration Time pHresistant resistant resistant itive (ppm) (hr) pH Bacteria 1 Bacteria 2Bacteria 3 None — 1 8.0 74 89 29 Sulfuric 200 1 2.8 50 22 <1 AcidSulfuric 250 0.5 2.6 17 33 1 Acid Sulfuric 250 1 2.6 <1 2 <1 AcidSulfuric 250 2.5 2.6 <1 <1 <1 Acid Sulfuric 300 0.5 2.6 8 1 <1 AcidSulfuric 300 1 2.4 <1 <1 <1 Acid

Example 6

Two membrane separation systems (systems A and B) each comprising apre-treatment unit and a membrane separation unit having a reverseosmosis membrane module of polyamide were driven for seawaterdesalination through reverse osmosis filtration to produce fresh water.In these, a culture of the acid-resistant bacteria having been obtainedin Reference Example 3 was added to the pre-treated seawater. Seawaterhaving been subjected to pH control to have a pH of from 3.5 to 4.0 waspassed through the both systems over a period of 30 minutes a day. Thesesystems thus having been subjected to pH control were more stablydriven, as compared with others not subjected to pH control. However,after these systems were continuously driven for 30 days in thatcondition, the pressure loss in the membrane separation unit increased.After this state, seawater having been subjected to pH control to have apH of 2.6 was passed through the system A over a period of 30 minutes aday, while seawater having been subjected to pH control to have a pH offrom 3.5 to 4.0 was passed through the system B also over a period of 30minutes a day. Through the system B, seawater having been subjected topH control to have a pH of 2.6 was additionally passed over a period of30 minutes a day, but once at intervals of 5 days. In those conditions,the two systems were continuously driven for 30 days. As a result, thepressure loss in the membrane separation unit in the two systems did notchange. While the systems were driven under the defined conditions, thenumber of living cells in the concentrate was counted. The number ofliving cells in the concentrate in the two systems decreased to{fraction (1/100)} or less, as compared with that in the concentrate inthose systems where only seawater having a controlled pH value of from3.5 to 4.0 was passed. The data are shown in Table 5. From Table 5, itis understood that the disinfecting effect of the seawater having acontrolled pH value of from 3.5 to 4.0 is not so good, but thedisinfecting effect of the seawater having a controlled pH value of 2.6is satisfactory. In addition, it is also understood that thedisinfecting effect of the seawater could be satisfactorily enhancedonly when the pH value of the seawater is lowered to 2.6 once atintervals of 5 days.

TABLE 6 Ratio of Living Microbes in Ratio of Sulfuric Treatment withAcid Water Concentrate Acid Used pH 3.5 to 4.0, 100 1 30 minutes a daypH 2.6, <1 2 30 minutes a day pH 3,5 to 4.0, <1 1.2 30 minutes a day pH2.6, 30 minutes a day (once at intervals of 5 days)

INDUSTRIAL APPLICABILITY

For disinfecting microorganisms that exist on and around membranes in amembrane separation apparatus for water purification, the method of theinvention is better than conventional methods of intermittently addinghighconcentration sodium bisulfite to the apparatus. According to themethod of the invention, microorganisms in the apparatus are all surelykilled.

What is claimed is:
 1. A method of bacteriostatsis or disinfection ofpermselective membranes in a membrane separation apparatus for waterpurification, comprising intermittently subjecting crude water to acidtreatment at a pH of 4 or lower during the operation of the membraneseparation apparatus, wherein the acid treatment is carried out for aduration of from about 0.5 to about 2.5 hours at intervals of at leastonce per week to maintain bacteriostatis or disinfection; and thenapplying the crude water to the membranes.
 2. The method ofbacteriostasis or disinfection for permselective membranes as claimed inclaim 1, wherein the crude water is subjected to acid treatement at a pHof 3.4 or lower.
 3. The method of bacteriostasis or disinfection forpermselective membranes as claimed in claim 2, wherein the crude wateris subjected to acid treatment at a pH of 2.6 or lower.
 4. The method ofbacteriostatis or disinfection for permselective membranes as claimed inclaim 3, wherein the frequency of the acid treatment is once atintervals of from 2 to 7 days.
 5. The method of bacteriostasis ordisinfection for permselective membranes as claimed in claim 1, whereinthe permselective membranes are reverse osmosis membranes.
 6. The methodof bacteriostasis or disinfection for permselective membranes as claimedin claim 1, wherein the crude water to be treated is seawater.
 7. Themethod of bacteriostasis or disinfection for permselective membranes asclaimed in claim 1, wherein the acid treatment is effected with at least120 ppm of sulfuric acid added to the crude water.
 8. A method forseparating or purifying water in a membrane separation apparatus,comprising the method of bacteriostasis or disinfection forpermselective membranes as claimed in claim
 1. 9. The method forseparating and purifying water as claimed in claim 8, wherein the crudewater to be treated is seawater.
 10. The method for separating andpurifying water as claimed in claim 9, wherein the crude water to betreated is previously subjected to intermittent disinfection withchlorine.
 11. The method for separating and purifying water as claimedin claim 8, wherein the crude water to be treated is previouslysubjected to intermittent disinfection with chlorine.
 12. Apre-treatment method comprising: providing a reverse osmosis membranetreatment apparatus comprising a first duct for feeding crude water to asand filter and means for intermittently subjecting the crude water toacid treatment at a pH of 4 or lower during the operation of thepre-treatment apparatus at time periods and durations effective tomaintain bacteriostatis or disinfection.
 13. The pre-treatment method ofclaim 12, wherein said apparatus further comprises a second duct forfeeding the crude water from the sand filter to a safety filter, a thirdduct for feeding the crude water from the safety filter to a reverseosmosis membrane treatment apparatus and a flocculant feeder for feedinga flocculant to the first duct.